The peloponnesian war began after
sparta attacked athens by land.
sparta attacked athens by sea.
athens attacked sparta by land.
athens attacked sparta by sea.
the answer is : sparta attacked athens by land.

Sparta attacked Athens by land.

You must note that the war was started by Sparta, and that Sparta had a better army, while Athens had a better navy. Since Sparta initiated the war, they were usually able to choose whether the battles would mostly be on land or in the sea. Sparta acted on them having a better land force, and invaded through land, which led to Sparta's victory.

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the deaths of over a million communist soldiers

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